Geoscience Reference
In-Depth Information
2
PRECIPITATION
LEARNING OBJECTIVES
When you have finished reading this chapter you should have:
An understanding of the processes of precipitation formation.
A knowledge of the techniques for measuring precipitation (rainfall and snow).
An appreciation of the associated errors in measuring precipitation.
A knowledge of how to analyse rainfall data spatially and for intensity/duration of a storm.
A knowledge of some of the methods used to estimate rainfall at the large scale.
An understanding of the process of precipitation interception by a canopy.
PRECIPITATION AS A PROCESS
Precipitation formation
The ability of air to hold water vapour is tempera-
ture dependent: the cooler the air the less water
vapour is retained. If a body of warm, moist air is
cooled then it will become saturated with water
vapour and eventually the water vapour will
condense into liquid or solid water (i.e. water or ice
droplets). The water will not condense spontane-
ously however; there need to be minute particles
present in the atmosphere, called condensation
nuclei , upon which the water or ice droplets form.
The water or ice droplets that form on condensation
nuclei are normally too small to fall to the surface
as precipitation; they need to grow in order to have
Precipitation is the release of water from the
atmosphere to reach the surface of the earth. The
term 'precipitation' covers all forms of water being
released by the atmosphere, including snow, hail,
sleet and rainfall. It is the major input of water to
a river catchment area and as such needs careful
assessment in any hydrological study. Although
rainfall is relatively straightforward to measure
(other forms of precipitation are more difficult) it is
notoriously difficult to measure accurately and, to
compound the problem, is also extremely variable
within a catchment area.
 
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